1. Field of the Invention
This invention relates to a magneto-optical recording medium and recording/reproducing apparatus used therefor, and more particularly to a magneto-optical recording medium having a recording layer and a reproducing layer so that microscopic magnetic domains can be recorded within the recording layer during recording and the magnetic domains thus recorded are magnified and transferred to the reproducing layer during reproduction, and recording/reproducing apparatus used therefor.
2. Description of the Related Art
There are magneto-optical recording mediums and recording/reproducing apparatuses of this kind disclosed as examples, e.g. in Japanese Laying-open Patent Publication No. H6-295479 (Oct. 21, 1994), G11B 11/10, Japanese Laying-open Patent Publication No. H8-7350 (Jan. 12, 1996), G11B 11/10, and so on.
The magneto-optical recording medium 10 includes a recording layer 14 and a reproducing layer 16 each formed by a magnetic layer on a substrate 12, as shown in FIG. 1. The recording layer 14 and the reproducing layer 16 have an intermediate layer 18 therebetween. A protecting layer 20 is formed on the recording layer 14. Incidentally, the intermediate layer 18 herein is formed by a non-magnetic layer, but can be by a magnetic layer. Meanwhile, the recording layer 14 and the reproducing layer 16 can be desirably formed of a known magnetic material.
Referring to FIG. 2, microscopic magnetic domains (hereinafter referred to also as "record magnetic domains") 22 are recorded within the recording layer 14 of this magneto-optical recording medium 10 by using a magnetic head (not shown.). During reproduction, the record magnetic domain 22 in the recording layer 14 is transferred to the reproducing layer 16 by irradiating a laser beam 24 as shown in FIG. 3. More specifically, the laser beam 24 has a temperature profile as shown in FIG. 3, wherein the temperature assumes a maximum at and close to a spot center and gradually decreases toward the outside. However, where the magneto-optical recording medium is for example an optical disc, the temperature profile on the magneto-optical recording medium is different in slant at between the frontward and the rearward with respect to a moving direction. That is, the slant is more abrupt at the rearward than the frontward. By utilizing such a temperature profile by the laser beam 24, the magneto-optical recording medium 10 is raised in temperature at only a desired point thereof.
Returning to FIG. 2(A), if a laser beam 24 is irradiated to the magneto-optical recording medium 10, the magneto-optical recording medium 10 is increased in temperature according to a temperature profile as shown in FIG. 3. Here, the reproducing layer 16 is formed of a magnetic layer assuming rich in sub-lattice magnetization of transition metals and as a magnetic thin film with perpendicular magnetization over a range from a room temperature to a Curie temperature Tc. Accordingly, when the laser beam 24 is irradiated, the reproducing layer 16 is increased in temperature and decreases in coercive force. This causes the record magnetic domain 22 of the recording layer 14 to be transferred, due to static magnetic coupling, through the intermediate layer 18 to the reproducing layer 16, thus forming a transferred magnetic domain (hereinafter referred also to as "seed magnetic domain") 26 within the reproducing layer 16. The transferred or seed magnetic domain 26 is formed at a location corresponding to the record magnetic domain 22. After forming the seed magnetic domain 26 within the reproducing layer 16, an external magnetic field Hep is applied thereto by a not-shown magnetic head, as shown in FIG. 2(B). This external magnetic field Hep is an alternating magnetic field, and applied for at least one period, preferably 2-4 period, while one minimum sized magnetic domain is passing through a hot spot 24a (see FIG. 3) formed by the laser beam 24. If the alternating or external magnetic field Hep applied is in a same direction (same polarity) as the transferred magnetic domain 26, the seed magnetic domain 26 is enlarged in magnetic-domain diameter to provide enlarged magnetic domains 26a and 26b, resulting in transfer of the record magnetic domain 22 through enlargement. If a reproducing laser beam is irradiated to the transferred magnetic domain 26 and the enlarged magnetic domains 26a and 26b by using an optical head (not shown), a state of magnetization in the reproducing layer 16, i.e. a record signal, is reproduced.
In such a magneto-optical recording medium, there is tendency of transfer error to occur as the size of the record magnetic domain 22 within the recording layer decreases in size. This is due to decrease of resolution as the transferred magnetic domain area 26 within the reproducing layer 16 becomes greater than the diameter of the record magnetic domain. On the other hand, the size of the transferred magnetic domain 26 of the reproducing layer 16 is determined by the size of a hot spot of the laser beam 24. In order to increase the record density by decreasing the size of the record magnetic domain 22, there is a necessity of decreasing the size of the hot spot 24a of the laser beam 24, that is, the size of the transferred magnetic domain 26 within the reproducing layer 16.
The laser beam has a temperature profile variable depending upon an output of the laser beam. Accordingly, the decrease in size of the hot spot 24a only require the reduction in output of the laser beam 24. However, the laser beam output has an effect upon reproducibility, which has to be taken into consideration for optimal setting.
In any of the prior arts, however, nothing has been considered as to optimize the laser beam output from such a point of view.